The use of fluidized bed furnaces for treating workpieces with chemically active gases is well known in the art. U.S. Pat. No. 3,749,805 of Karl H. Seelandt entitled FLUID BED FURNACE is an example of such prior art furnaces. In such furnaces, a bed of finely divided solid refractory particles is disposed within a vessel and a gas is directed through the particle bed from the lower portion of the vessel causing the particles to migrate in the manner of a fluid. The workpiece, is suspended in the fluidized bed of solid particles, and an atmosphere of the proper gas to produce the desired chemical reaction is maintained in the bed. In addition, the bed is provided with a source of heat and functions as a heat transfer medium to maintain the temperature of the work piece at a suitable temperature for the desired chemical reaction.
U.S. Pat. No. 4,623,400 of Joseph E. Japka, Robert Staffin and Swarnjeet S. Bhatia entitled Hard Surface Coatings for Metals in Fluidized Beds is an example of the devices of the prior art for treating work pieces in fluidized beds. The reaction vessel of this patent has a horizontal perforated distribution plate adjacent to the bottom thereof which supports a bed of refractory particles, and these particles are maintained in a fluid state by a flow of inert gas into a plenum disposed directly below the distribution plate. A second and chemically active gas is introduced directly into the fluidized bed through a separate conduit.
U.S. Pat. No. 4,512,821 of Robert Staffin, Carol A. Girrell and Mario A. Fonzoni entitled Method for Metal Treatment Using a Fluidized Bed discloses a similar reaction vessel in which a chemically active gas is mixed with an auxiliary gas to provide the flow for fluidization of the bed and establishes the proper gas atmosphere within the reaction vessel. U.S. Pat. No. 4,461,656 of John A. Rose entitled Low Temperature Hardening of the Surface of a Ferrous Metal Workpiece in a Fluidized Bed Furnace also fluidizes a bed of refractory particles with a mixture of chemically active and inert gases.
Carburizing is one of the processes conventionally carried out in a fluidized bed furnace. In one carburizing process, hydrocarbon bearing gases are introduced with a suitable inert carrier gas into the fluidized bed. This process has proven to be unreliable and unrepeatable, and produces excessive free carbon, or soot, rather than the carbon monoxide necessary for a reliable process.
An endothermic gas generator produces a carbon/oxygen containing gas suitable for the carburization process. In this reaction, a hydrocarbon containing gas, such as natural gas which generally contains CH.sub.4, is combined with air while supplying heat, according to the following formula: EQU 0.29CH.sub.4 (gas)+0.71 air=0.29CO (gas)+0.56H.sub.2 +0.56N.sub.2,
and produces reaction products in volumetric proportion as follows:
______________________________________ Carbon Monoxide (CO) 20% Hydrogen (H.sub.2) 39% Nitrogen (N.sub.2) 40% Water Vapor &lt;1% Carbon Dioxide (CO.sub.2) Trace Oxygen (O.sub.2) Trace ______________________________________
Endothermic gas is stable and suitable for the carburizing process, but endothermic gas generators produce gas at around atmospheric pressure, thereby requiring pressurizing of the gas or the use of an auxiliary gas booster before it can be used in a fluidized bed reactor.
The use of an inert gas plus methane for carburizing is not desirable because insufficient carbon monoxide is generated to allow the carburizing process to take place. The methane breaks down to basically solid carbon and this diffuses into the steel. This is a very slow and unreliable process. Experiments have shown that if an activator such as barium carbonate is added to a fluid bed, using an inert gas plus methane, the carburizing process increases in speed and uniformity. This is the result of carbon monoxide from the activator being generated. This is well known to those skilled in the art of pack or solid carburizing.
Endothermic gas contains the carbon monoxide necessary for carburizing and additions of methane react with the water vapor and carbon dioxide present to allow the carburizing process to occur. Water vapor and carbon dioxide are decarburizers to the steel and hence must be lowered before a sufficient carbon potential will occur so carburizing will take place. It is therefore an object of the present invention to provide a gas generator capable of producing a sufficient flow of gas at a sufficient pressure to make it unnecessary to utilize an inert gas for fluidization of the bed of the furnace.
It is an object of the present invention to provide an endothermic gas generator which will produce a sufficient flow of gasses at a sufficient pressure, including carbon/oxygen bearing gasses, to directly fluidize the bed of a fluidized bed furnace.
The prior art teaches the use of gas pressure boosters, carburetors, mixers or blenders between and/on an endothermic gas generator and a fluidized bed furnace in order to provide sufficient gas pressure to fluidize the bed of the furnace. Such components increase the cost of a combination endothermic gas generator and fluidized bed furnace. Therefore, it is an object of the invention to provide a combination endothermic gas generator and fluidized bed furnace which does not require a gas pressure booster, carburetor, mixer or blender; and which reduces the cost of a combination endothermic gas generator and fluidized bed furnace.